The present study is designed to provide new information concerning the nature of microtubule-microtubule interaction, with particular emphasis on the role of microtubule-associated proteins (MAPs) in this process. The overall objective of the proposed research is to study the manner in which microtubules and their associated high molecular weight filamentous proteins interact with one another to produce a cellular cytoskeletal network. Initially, a combination of biochemical, morphological, and immunological techniques will be employed to show that microtubule-associated proteins are important components of microtubules in vivo as well as in vitro. The manner in which the MAPs are associated with each other and with tubulin will also be determined to increase our knowledge of the ultrastructure of the microtubule. These in vitro studies will then be related to the information obtained from experiments on the microtubule marginal band system of nucleated erythrocytes. Since the marginal band is composed of a highly crosslinked bundle of microtubules that can be easily isolated intact, the system provides ideal experimental material for the biochemical and morphological characterization of an integrated system of microtubules.